1. Field of the Invention
The present invention relates to a laser processing method using pulse laser, in particular, using a ultra-short pulse laser whose pulses have a temporal width of 10−12 second or less such as femto second (10−15 second) pulses, and to a processing apparatus for performing the processing method.
2. Description of the Related Art
Recently, there is rapidly growing interest in a processing technology making use of a pulse laser, in particular, ultra-short pulse laser having femto second pulses. However, when the ultra-short pulse laser having a wavelength band width is focused using an ordinary refraction single lens, chromatic aberration occurs due to the refractive dispersion of a refraction lens (a property that the refractive index depends on wavelength, thereby a focused spot is extended laterally and longitudinally. The chromatic aberration depends on the wavelength bandwidth of the ultra-short laser pulse and the magnitude of the dispersion of the refraction lens.
Further, the pulse width of the ultra-short pulse laser is extended by the dispersion of the refraction lens. The magnitude of the pulse extension depends on the peak intensity of pulses, in addition to that it depends on the wavelength bandwidth of the pulse and the dispersive characteristics of the lens material.
Accordingly, when the ultra-short pulse laser is applied to processing, it is necessary to make the processing practically usable by correcting chromatic aberration, or by correcting both chromatic aberration and pulse extension in some cases.
Further, as shown in, for example, FIG. 1, there is also known a processing method of branching the pulse beam from a laser generator 1 into a plurality of pulse beams using a diffraction element 2, focusing the plurality of branched pulse beams using a refraction type focusing lens 3 disposed behind the diffraction element 2, and impinging the focused pulse beams on a work 4. The diffraction element 2 is designed such that the phase thereof is distributed to provide the respective branched beams with substantially the same intensity.
Further, the focusing lens 3 has a Fourier transform action and focuses the plurality of branched pulse beams. In general, there is the following relationship between a pulse width Δt of a pulse and a wavelength width Δλ of a pulse.Δt·Δλ≧C  (1)where, C shows constant. It can be found from the expression (1) that a pulse having a shorter pulse width has a larger wavelength bandwidth. Note that for femto second pulses, a 100 femto second pulse has a wavelength band width of up to ±10 nm.
When the wavelength band width of a pulse is shown by Δλ, the positional derivation (chromatic aberration) Δh of laser beams branched by the diffraction element 2 can be estimated by the following relation (2).Δh=(mf/p)Δλ  (2)where, m shows the order of diffraction, f shows the focal length of a lens, p shows the length of one cycle of the diffraction element 2. Since the pulse beams branched by the diffraction element 2 are extended as described above, the diameter of a hole tobe processed at the position where the branched beam is focused is also extended in the diffraction direction. While the case in which the diffraction element is arranged one-dimensionally is described here, this is also applied to the case in which it is arranged two-dimensionally.
It can be understood from the relation (2) that when the femto second pulse is branched using the diffraction element, it is difficult to focus the pulses branched by the diffraction element into a small spot because a large chromatic aberration occurs due to the large wavelength band width of the femto second pulse. In contrast, in a long pulse, no problem with the above chromatic aberration occurs even if the pulse beam is branched by the diffraction element because the wavelength band width of the pulse is negligible.
When the femto second pulse laser is branched into the plurality of pulse beams using the optical system shown in FIG. 1 and the pulse beams are focused, the focused spots of the higher-order diffractived beams are extended in terms of shape in the diffraction direction as shown in FIG. 2 due to the wavelength band width of the pulse. Therefore, when the array of the focused spots is impinged on the work 4, processed marks due to higher-order beams are deformed in an oval shape different from the shape of processed marks from lower-order beams. Accordingly, it is difficult to perform the processing uniformly at a plurality of positions.
The femto second laser pulse used in the experiment of branching and focusing using the optical system shown in FIG. 1 has a pulse width of 100 fs, a pulse energy of 1 mJ (repetition rate of 1 KHz), a center wavelength of 800 nm, and a half wavelength bandwidth of ±10 nm.